The goal of this research program is to elucidate the nature of the interaction between a steroid-sensitive neural circuit and the DAergic system. This interaction mediates, in part, the activation of masculine reproductive behavior. DA and testosterone are well-known to play a stimulatory role in the control of male reproductive behavior. It is, however, unclear how the activational effects of DA relate to the regulation of male sex behavior by testosterone. It is hypothesized that DA is part of the biochemical cascade initiated by testosterone that ultimately results in male copulatory behavior and that DA may facilitate male sexual behavior by increasing the behavioral effectiveness of testosterone. The proposed work capitalizes on a number of recent findings concerning the quail preoptic area (POA) which makes this a unique model: The quail POA contains a sexually dimorphic and testosterone-sensitive nucleus, the medial preoptic nucleus (POM), which provides a clearly defined morphological definition of the area critical for the action of testosterone on sexual behavior. The testosterone-metabolizing pathways (in particular its aromatization) mediating the activation of behavior have been described and an immunocytochemical (ICC) procedure has been developed that permits the visualization of aromatase-containing cells in the quail POA. DA turnover is sexually differentiated in the quail POM (higher turnover in males). Finally, in quail there is evidence that the DAergic system regulates aromatase activity in the POA. A series of 11 experiments are proposed that are designed to analyze four specific aspects of the DA action in the activation of male sexual behavior. The first set of experiments will characterize the central mechanisms controlling anticipatory aspects of sexual behavior and thereby provide a comparison with those mechanisms activating the consummatory aspects. A second set of experiments will have the goal of characterizing the DAergic inputs to the POM and nucleus Accumbens of the Stria Terminalis (Ac-nST) complex and their relation to the activation of anticipatory and consummatory aspects of male sexual behavior. The third series of experiments will investigate whether the sexual dimorphism in behavior and the dimorphism in preoptic aromatase activity can be explained, at least in part, by the dimorphism in DAergic activity in the POM. Finally, the fourth series of experiments will ask whether DA stimulates male sexual behavior by modulating the action of testosterone (changes in aromatization) or by acting on other steroid-dependent or -independent mechanisms.